1. Field of the Invention
The present invention relates to a dielectric filter, which comprises an internal-conductor hole formed inside a dielectric block and an external conductor formed on the outer surface thereof, a dielectric duplexer, and a communication apparatus using the filter and duplexer.
2. Description of the Related Art
The construction of a conventional dielectric filter with a balanced-to-unbalanced transform function using dielectric block will now be described with reference to FIGS. 8A and 8B.
FIG. 8A is an exterior perspective view of a dielectric filter facing up. FIG. 8B is a sectional view taken along the plane passing through the holes. In this example, the inner diameters of the internal-conductor holes have a stepped structure wherein the ends of each internal-conductor hole have an inner diameter of D1 and the central portion of each hole has an inner diameter of D2.
In FIGS. 8A and 8B, reference numeral 1 denotes a dielectric block. Inside the dielectric block 1, internal-conductor holes, indicated by 2a and 2b, are provided, and both ends thereof are open. The inner surfaces of the internal-conductor holes have internal conductors 3a and 3b formed thereon. Also, the outer surface of the dielectric block 1 is formed with external conductors on four surfaces, but not on the end surfaces at which the open ends of the internal-conductor holes 2a and 2b are formed. Balanced terminals 5 and 6 which are coupled respectively to the open ends of one of the internal conductors 3a and an unbalanced terminal 7 which is coupled to one of the open ends of the other internal conductor 3b are formed on the outside of the dielectric block 1 and isolated from the external conductor 4.
In such a conventional dielectric filter, there are problems to be solved, such as those described below.
For example, in a dielectric filter with a balanced-to-unbalanced transform function, the number of external terminals associated with each internal conductor differs, for example, two external terminals may be provided on the balanced side and one external terminal may be provided on the unbalanced side. Thus, although the resonance frequencies of the respective internal conductors should match, the resonance frequency of the internal conductor on the balanced terminal side is higher than the resonance frequency of the internal conductor on the unbalanced terminal side.
Also, in a case where the characteristic impedance of a line coupled to an external source differs between the balanced terminal side and the unbalanced terminal side, the necessary coupling capacitance does not match between the balanced terminal side and the unbalanced terminal side. As a result, degradation of filter characteristics, for example, an increase in reflection loss, occurs, and signals deteriorate during the balanced-to-unbalanced conversion.
The present invention addresses these problems and provides a dielectric fitter, in which the mismatch of the resonance frequency between the balanced terminal side and the unbalanced terminal side is corrected, and a desired external coupling capacitance can be obtained so that even if the characteristic impedance of a line coupled to an external source differs from a preferred characteristic impedance, superior reflection loss characteristics can be obtained in such a manner as to compensate for the impedance. The invention also provides a dielectric duplexer; and a communication apparatus having either or both of the filter and duplexer.
To this end, in one aspect, the present invention provides a dielectric filter provided with a plurality of internal-conductor holes, each having an internal conductor formed on the surface thereof inside a dielectric block which has a substantially rectangular parallelepiped shape, having both ends or the portions in the vicinity of both ends formed as open ends, and having an external conductor formed on the outer surface of the rectangular parallelepiped. The dielectric filter comprises a pair of balanced terminals, which are coupled respectively to the portions in the vicinity of the open ends of the internal conductor formed in a predetermined one of the internal-conductor holes; and an unbalanced terminal which is coupled to the vicinity of one of the open ends of the internal conductor formed in another one of the internal-conductor holes. The internal-conductor hole in which is formed the internal conductor to which the balanced terminal is coupled and the internal-conductor hole in which is formed the internal conductor to which the unbalanced terminal is coupled are differently sized in cross-section. With this construction, the resonance frequency of the internal conductor which is coupled to the balanced terminal and the resonance frequency of the internal conductor which is coupled to the unbalanced terminal, which differ in the prior art, are made to match. Furthermore, the external coupling capacitance can be set as desired.
The internal-conductor holes may be formed into a stepped structure made up of plural layers, in which the size of the cross section of the internal-conductor hole in the inside of the dielectric block differs from the size of the cross section in a portion in the vicinity of each open end, and the sizes of the cross sections in at least one of the layers of the internal-conductor hole in which is formed the internal conductor to which the balanced terminal is coupled and the internal-conductor hole in which is formed the internal conductor to which the unbalanced terminal is coupled are different. With this construction, the resonance frequencies which are different between the internal conductor which is coupled to the balanced terminal and the internal conductor which is coupled to the unbalanced terminal are made to match. Furthermore, the external coupling capacitance can be set as desired.
The internal-conductor holes may be formed into a tapered structure in which the size of the cross section of the internal-conductor hole increases from the inner portion toward the opening, and the sizes of the cross sections at a given distance from the openings at one end, of the internal-conductor hole in which is formed the internal conductor to which the balanced terminal is coupled, and the internal-conductor hole in which is formed the internal conductor to which the unbalanced terminal is coupled, are different. With this construction, the resonance frequencies which are different between the internal conductor which is coupled to the balanced terminal and the internal conductor which is coupled to the unbalanced terminal are made to match, and the external coupling capacitance can be adjusted as desired. Moreover, the molding of internal-conductor holes becomes easier.
In another aspect, the present invention provides a dielectric filter provided with a plurality of internal-conductor holes, each having an internal conductor formed on the surface thereof inside a dielectric block which has a substantially rectangular parallelepiped shape, having both ends or portions in the vicinity of both ends being formed as open ends, and having an external conductor formed on the outer surface of the rectangular parallelepiped. The dielectric filter comprises a pair of balanced terminals which are each coupled to one respective portion in the vicinity of one of the open ends of the internal conductor formed in a predetermined one of the internal-conductor holes; and an unbalanced terminal which is coupled to a portion in the vicinity of one of the open ends of the internal conductor formed in another one of the internal-conductor holes. An external-conductor-removed section in which the external conductor is partially removed is provided in the vicinity of the other open end of the internal-conductor hole in which is formed the internal conductor to which the unbalanced terminal is coupled. With this construction, the resonance frequencies which are different between the internal conductor which is coupled to the balanced terminal and the internal conductor which is coupled to the unbalanced terminal are made to match.
In the dielectric filter of the present invention, preferably, the sizes of the cross sections of the internal-conductor hole in which is formed the internal conductor to which the balanced terminal is coupled, and the internal-conductor hole in which is formed the internal conductor to which the unbalanced terminal is coupled are different. Or the internal-conductor holes may be formed into a stepped structure made up of plural layers, in which the size of the cross section of each internal-conductor hole in the inside of the dielectric block differs from the size of the cross section in a portion in the vicinity of an open end, and the sizes of the cross sections in at least one of the layers of the internal-conductor hole in which is formed the internal conductor to which the balanced terminal is coupled and the internal-conductor hole in which is formed the internal conductor to which the unbalanced terminal is coupled are different. Or the internal-conductor holes may be formed into a tapered structure in which the size of the cross section of the internal-conductor hole increases from the inner portion toward an opening, and the sizes of the cross sections in the inside at the same distance from the opening of the internal-conductor hole in which is formed the internal conductor to which the balanced terminal is coupled and the internal-conductor hole in which is formed the internal conductor to which the unbalanced terminal is coupled are different. In any of the foregoing structures, an external-conductor-removed section in which an external conductor is partially removed may also be provided in a portion in the vicinity of an open end of the internal-conductor hole in which is formed the internal conductor to which the unbalanced terminal is coupled. With this construction, similar to that described above, the resonance frequencies which are different between the internal conductor which is coupled to the balanced terminal and the internal conductor which is coupled to the unbalanced terminal are made to match, and the external coupling capacitance can be adjusted as desired.
On the outer surface of the dielectric filter of the present invention, preferably, a floating electrode, which is separated from the external conductor by the external-conductor-removed section, is formed in a portion in the vicinity of the open end on the unbalanced terminal side. With this construction, the resonance frequencies which are different between the internal conductor which is coupled to the balanced terminal and the internal conductor which is coupled to the unbalanced terminal are made to match.
In another aspect, the present invention provides a dielectric duplexer formed of a plurality of dielectric filters and having predetermined filtering characteristics.
In another aspect, the present invention provides a communication apparatus formed by using the dielectric filter and/or the dielectric duplexer. As a result, a compact and lightweight communication apparatus is obtained.
The above and further aspects and novel features of the invention will become more fully apparent from the following detailed description when read in conjunction with the accompanying drawings.